This was so much fun! It took me some time, but I was able to beat it eventually.
Some things confused me, though.
One was that the aliens would stop moving if I knocked them backward into another alien. So two could gang up on me in a line and I couldn't stop them. And the larger spiders in the "get food rations" room were not affected by the gun at all. Was that all intentional?
The other was changing spacesuits. Did that have an effect other than changing the sprites?
The design doc in your source was incredible. Well done!